Helium occurs in very low concentrations in certain natural gas fields. Natural gas streams from which helium can be economically recovered typically contain approximately 0.1% to 0.5% helium. This helium must be upgraded to produce a crude helium stream containing typically at least 30% helium.
Producing a crude helium product stream is usually done in two or more successive upgrading steps. The first upgrading step generally produces a crude helium stream containing about 1 to 10% helium, and successive upgrading steps are required to boost the helium content of this stream to 30% or greater.
Due to the high value of the helium, high recovery is usually required. Achieving the high recovery as the helium content is increased from 1 to 10% up to 30% or greater has in the past required the addition of compression machinery. A process which could achieve high helium recovery without the need for additional compression machinery would therefore represent an improvement over the current practice.
In addition to producing crude helium, a helium upgrading process is typically required to also produce a high purity nitrogen stream to be used for cold box purge. The ability of the process to produce this additional product stream with a minimum of added equipment would be a further advantage.
The current practice for producing a crude helium product stream (i.e; &gt;30% helium) includes the multi-stage flash process and the distillation process. Each of these processes requires additional compression to achieve high helium recovery.
In the flash cycle, which is disclosed in U.S. Pat. No. 3,260,058, feed gas is partially liquefied and phase separated. The vapor thus produced contains about 90% or more of the helium contained in the feed stream. Helium which remains dissolved in the liquid is recovered by subsequent flash steps in which helium-rich vapors are flashed off. These vapors are combined, rewarmed, compressed back to feed pressure and mixed with the feed gas so the helium can be recovered.
In the distillation process, which is disclosed in "A New Approach to Helium Recovery", Kellogram Issue No. 3, M. H. Kellogg Co., 1963, feed gas is partially condensed and fed to a distillation column which produces a helium-rich vapor product stream containing at least 99% of the helium in the feed gas. A heat pump compressor is used to supply reboil to the bottom of the column by condensing high pressure heat pump fluid and reflux to the top of the column by boiling low pressure heat pump fluid.
In each of these cases, additional compression is required to achieve high helium recovery.